An electric discharge machining apparatus is an apparatus that performs machining of a workpiece by generating arc discharge between a machining electrode and the workpiece. In the electric discharge machining apparatus, a power source (a machining power supply) for generating the arc discharge is necessary. Concerning the configuration of the machining power supply, there have been various configurations.
Among the configurations, there is known a capacitor discharge system that makes use of electrode-gap capacitor capacitance present in parallel to an electrode gap. A basic configuration is configured by a power supply, a current limiting resistor, and electrode-gap capacitor capacitance. When a capacitor element is connected to the electrode gap in parallel, the electrode-gap capacitor capacitance is capacitance obtained by combining mechanical stray capacitance between an electrode and a workpiece and the capacitance of the capacitor element. When the capacitor element is not connected to the electrode gap in parallel, the electrode-gap capacitor capacitance is mechanical stray capacitance between the electrode and the workpiece.
In the basic circuit, first, when the electrode-gap capacitor capacitance is charged from the power supply via the power supply limiting resistor, an electrode-gap voltage rises. When the electrode-gap voltage exceeds a discharge start voltage, dielectric breakdown occurs. Electric charges accumulated in the electrode-gap capacitor capacitance are used as discharge energy. The electrode-gap voltage drops to arc potential according to electric discharge. An electric current flows by the discharge energy stored in the electrode-gap capacitor capacitance. In this case, because an electric current from the power supply is limited by a current limiting resistance value, a ratio of the electric current from the power supply is extremely small in the electric current flowing to the electrode gap at the start of electric discharge. Therefore, when the electric charges stored in the electrode-gap capacitor capacitance are exhausted, it is difficult to continue the electric discharge any more. Thereafter, when the electric discharge ends and insulation recovery occurs, an electric current flows to the electrode-gap capacitor capacitance again from the power supply via a charging resistor, and the electrode-gap voltage rises.
A phenomenon in which ON (electric discharge) and OFF (non-electric discharge or open) are repeated according to the behavior of the electric discharge itself is called self-excited oscillation. In the self-excited oscillation, a discharge current is specified by the electrode-gap capacitor capacitance. However, by inserting a switching element between the power supply and the electrode gap, the discharge current can also be specified by switching.
Patent Literature 1 mentions that, in an electric discharge machining power supply circuit, a variable resistor and a switching element, and a current suppression resistor having a resistance value sufficiently larger than a resistance value of the variable resistor are connected in parallel between a direct-current voltage source and a machining electrode, the switching element is turned on when machining is stably in progress, and the switching element is turned off when the machining becomes unstable. Consequently, according to Patent Literature 1, when the machining becomes unstable, supply energy between the machining electrode and a workpiece is reduced to suppress occurrence of an electric discharge phenomenon. Therefore, it is possible to reduce electric discharge in a side surface direction and secondary electric discharge that occurs via machining powder. It is possible to attain stabilization of electric discharge machining and improvement of machining speed.
Patent Literature 2 mentions that, in a wire electric discharge machining apparatus, a direct-current power supply and an electrode gap, in which a workpiece and a machining electrode are arranged to be opposed to each other, are full bridge-connected by four switching elements and a group pulse pattern is supplied to the switching elements in one diagonal arm among the four switching elements to switch the switching elements. In this case, when detecting electric discharge on the basis of a signal from an electrode-gap state detection circuit, a control unit sets a pulse pause time after electric discharge detection longer than a pulse pause time during non-electric discharge. When further detecting a state in which electric discharge tends to continue and lead to disconnection of a wire, the control unit sets a pulse pause time longer than the pulse pause time after the electric discharge detection. Consequently, according to Patent Literature 2, a pause time of a group pulse is controlled according to an electric discharge state during electric discharge. Therefore, it is possible to prevent concentration of the electric discharge and attain stabilization of machining and improvement of machining speed.
On the other hand, it is also possible to adjust ON and OFF of self-excited oscillation by inserting an inductance element and a capacitor element in a power supply circuit other than a resistor.
Patent Literature 3 mentions that, in a wire cut electric discharge machining apparatus, a variable resistor is provided in a wiring before a contact of an electromagnetic switch and an inductance element is provided in a wiring extended from the contact of the electromagnetic switch to a workpiece. Consequently, according to Patent Literature 3, a circuit that supplies very small electric discharge energy to a machining gap is configured by the variable resistor and the inductance element. The inductance element also plays a role for cancelling a stray capacitor component in the wire.